"It has also been plausibly argued that the Great Migration of millions of African Americans from the rural South to the factories in the North would not have happened if factory owners had been able to hire the European migrants they preferred."

In context, this is more than just an observation of potential cause and effect; the whole paragraph is about revising his previously-advocated policy positions: he used to think immigration was good, but he now sees a correlation between equality and low immigration. So what policy did he advocate re racist hiring policies at factories before, and what has it been changed to? Is he saying he used to be opposed to letting Detroit factory owners hire only whites, but now he thinks they should have been allowed to, because the Great Migration was a bad thing? Or is he saying he used to support letting Detroit factory owners hire only whites, but now he opposes it because the Great Migration was a good thing?

EDIT: OK, I think he was actually saying something else: the Great Migration happened in part because factories needed black workers; they wouldn't have hired black workers if immigration from Europe had been as open as it had been previously. So the Great Migration was good, and that good was partially caused by a clamp-down on migration.